Poly(ethylene terephthalate) (PET) is widely used as an extrusion and injection-molding resin for the fabrication of various articles for household or industrial use, including appliance parts, containers, and auto parts. PET also is commonly extruded into sheet (including film) of various thicknesses, which may be used as-fabricated or shaped, e.g., by thermoforming, into articles such as display articles, signs, or packaging articles. For example, extruded PET sheet material can be used to make trays, packages or containers in which foods may be both stored and heated and/or cooked. As used herein, the terms “tray” and “trays” include packages and containers in which food is packaged and sold for subsequent heating and/or cooking while still in the tray, package or container. Food trays fabricated from crystallized PET (CPET) retain good dimensional stability over the range of temperatures commonly encountered during both microwave and conventional oven cooking (known as dual ovenable).
The manufacture of thin-walled containers (trays) using the thermoforming process is well-known in the art. Such polyester food trays typically are manufactured by first extruding a sheet of polyester, then thermoforming the tray in a heated mold. Specific processes for extruding polyester sheeting and thermoforming the sheet material to produce CPET food trays are also well known, for example, as described by Siggel et al. in U.S. Pat. No. 3,496,143. The thermoforming process both forms the shape of the tray and crystallizes the polyester resin. The manufacture of this type of polyester article requires that it be initially formed from substantially amorphous polyester sheet. Crystallization is then accomplished by means of holding the polyester at a temperature between its glass transition temperature (Tg) and the crystalline melting temperature (Tm) while in the mold. Crystallization of the sheet in its final shape produces the desired high temperature stability of the thermoformed article. The sheet material used may be prepared in a process separate from the thermoforming process (sometimes referred to as the roll-fed or in-line process) which uses sheet heated from below the glass transition temperature. Alternatively, the sheet material may be prepared in-line with the thermoforming process such that the melt may not vitrify or vitrify completely before contact with the mold (sometimes referred to as the melt-to-mold process).
Unmodified PET crystallizes slowly when cooled from the melt or heated from below the glass transition temperature. To obtain acceptable manufacturing economics, it is necessary that the rate of thermal crystallization be rapid. A well-known method to increase the rate of crystallization is to incorporate a crystallization nucleator into the polyester. These crystallization rate enhancers typically are inorganic or organic solids finely dispersed throughout the polyester. Such nucleators typically are used at a concentration, relative to the polyester being nucleated, of at least 0.05% by weight. The great majority of prior art that attempts to increase the crystallization rate of polyester articles is concerned with crystallization during cooling of the polymer, especially in injection molding processes. In crystallization during cooling of the polymer from the melt, one measure of improvement of the crystallization characteristics is an increase in the temperature at onset of crystallization. One characteristic of typical crystallization nucleators well-known in the art, such as talc, is that they promote crystallization during cooling from the melt as well as during heating from below the glass transition temperature. For example, an article injection molded from typical nucleated polyester crystallizes to at least some degree while in the injection mold. This is desirable if the object is to produce a crystalline injection molded part. If, however, the object is to produce an amorphous part, such as an extruded sheet, crystallization from the melt is objectionable because it may interfere with subsequent operations, such as thermoforming. The best additives for enhancing the processing of thermoforming heat-stable, crystalline parts by thermoforming therefore are those that will enhance crystallization on heating from below the glass transition temperature, and ideally have little or no enhancement (or even supression) of crystallization rate when cooling from the melt.
Fast crystallization rates are not the only consideration for successful implementation of CPET for food containers, however. One problem encountered with polyester food trays is that they can suffer from poor impact properties, especially at low temperatures. The impact properties of food trays may be affected detrimentally by the presence of some nucleating agents, especially inorganic nucleating agents. One way to improve the impact properties (toughness) of these articles is to use high molecular weight polyester in the fabrication of the tray. Therefore, polyester used in food trays often is specially manufactured to produce intrinsic viscosities (IVs) of about 0.90 to about 1.05 dL/g. Another approach is to add an impact modifier to the polyester composition. In general, trays are toughest when both approaches are utilized. The presence of majority amounts of polyester in the composition provides the other necessary properties such as tensile strength, stiffness and temperature resistance. Of the impact modifiers used in polyester compositions, terpolymers based on ethylene, an alkyl acrylate and glycidyl acrylate, or blends of similar polymers provide an attractive combination of properties in this application, e.g., as disclosed by Epstein in U.S. Pat. No. 4,172,859 and Deyrup in Published PCT Application WO 85/03718, though other impact modifying agents may be used.
The crystallization rate of PET and other polyesters has been increased through the use of additives. See, for example, Gachter and Miller, in Plastics Additives, Chapter 17, Hanser Publications, 1992. Several different mechanisms have been proposed to explain the activity of these additives. In the majority of cases wherein an additive is included in a polyester, the objective is to improve the crystallization rate as the melt is cooled, which is especially significant for injection molding applications. The difference in the crystallization rate of polymers, depending upon whether they are heated from below the glass transition temperature to the crystallization temperature or are cooled from the melt to the crystallization temperature, has been described by D. W. van Krevelen, Chimia, 32, (1978), p. 279. This reference describes large differences in the nucleation density depending upon thermal history. Little prior art exists specifically outlining the improvement of crystallization rate on heating from the glass rather than cooling from the melt. Polymers that quickly crystallize from the glass while crystallizing relatively slowly from the melt are advantageous for the thermoforming process.
Polyester compositions suitable for thermoforming in short cycle times are described by Muschiatti et al., U.S. Pat. No. 5,405,921. The nucleating agents used by by Muschiatti et al. are comprised of metal salts of polyesters, preferably formed before contact with the polyester. These compositions are claimed to be useful for thermoforming. Kinami et al., U.S. Pat. No. 5,567,758, claim polyester compositions with specific endgroup chemistry together with inorganic compounds that produce moldings that crystallize quickly at low temperatures. Logullo, U.S. Pat. No. 5,102,943, discloses thermoformed tray compositions containing a small amount of sodium ion and wax. The Logulo patent describes good crystallization rates due to inclusion of the sodium ion and good trimming and stacking properties due to inclusion of the wax.
Research Disclosure 30655 (October 1989) describes a highly crystallizable polyester blend, which contains poly(butylene terephthalate), a copolyetherester, or nylon 6,6 to reduce the induction time of the crystallization process. Richeson et al., U.S. Pat. No. 4,996,269, describe a polyester composition suitable for thermoforming thin walled articles wherein the composition consists of poly(ethylene naphthalate) and from 1 to 6 weight percent of a polyester elastomer. The difference in temperature between the crystallization peak and melting peak was increased due to the presence of the polyester elastomer.
There have also been many reports of the use of poly(oxytetramethylene)diol as a nucleating additive for PET. As early as 1972, there were reports in the patent literature, e.g., U.S. Pat. No. 3,663,653, of the copolymerization of poly(oxytetramethylene)diol in PET for the purpose of increasing crystallization rate. Nield, U.S. Pat. No. 4,322,335, describes a polyester composition containing a block copolyester with a “crystallizaton nucleant” consisting of an ethylene-based ionomer. The block copolyester is described as consisting of ethylene terephthalate units and polymeric units having a glass transition temperature of less than 0° C., including polyethers such as polyethylene glycol, polypropylene glycol, polytetramethylene glycol, and polydecamethylene glycol and polyesters such as poly(ethylene adipate), poly(tetramethylene adipate), polycaprolactone, poly(ethylene sebacate), poly(ethylene azelate), and poly(oxy-diethylene sebacate). In addition, Garrison, U.S. Pat. No. 4,548,978, describes multi-component nucleating systems for PET, consisting of poly(oxyalkylene)diols, though most of the Garrison's focus is on poly(oxyethylene)diol. Garrison describes a complementary effect for the use of polyalkylene glycols with plasticizers. The formulations of the '978 patent also contain a “crystallization promoter” consisting of organic ionic hydrocarbon copolymers.
The use of poly(tetramethylene terephthalate), also known as polybutylene terepthalate or PBT, as a nucleating agent in PET was investigated by Misra et al., Journal of Polymer Science, Polymer Physics Edition, 24, (1986), page 983. The authors prepared PET/PBT block copolymers with a small fraction of the total copolymer consisting of the PBT. At concentrations less than 3.5% PBT, the crystallization rate (measured at 95° C.) increased, though for higher concentrations than this the crystallization rate again decreased, the rates remained higher than unmodified PET. This effect was attributed to an entropy reduction for the PET chains when the PBT blocks crystallize, which in turn lowers the free energy for crystallization of the PET. This effect was not observed for random PET/PBT copolymers.
None of the prior art publications cited above address the improvement of impact resistance of the polyester or the desirability of enhancing crystallization rate only from the glass, and not during cooling from the melt.
Many publications describe the use of additives to improve the toughness of polyester compositions, and the use of modified polyolefin copolymers as impact modifiers for polyesters is well-known. Epstein, U.S. Pat. No. 4,172,859, discloses multiphase thermoplastic compositions where one phase contains 60-99 weight percent of a polyester matrix resin, and the remaining phase or phases contains a random copolymer. This copolymer is adhered to the matrix resin, and has a tensile modulus of 0.07 to 1378 bar (1.0-20,000 psi), or less than 1/10th that of the matrix resin. Among the random copolymers disclosed and exemplified by Epstein in such compositions are poly(ethylene/methyl acrylate/glycidyl methacrylate) and poly(ethylene/vinyl acetate/glycidyl methacrylate). These compositions provide enhanced toughness as compared to unmodified polyester polymers.
Published PCT Application WO 93/15146 discloses the use of various non-functionalized poly(alkyl acrylates) to toughen food trays made from CPET. These poly(alkyl acrylate) materials may be used alone or in combination with an acrylic core-shell impact modifier. U.S. Pat. No. 5,382,628 discloses the use of polyester or copolyester additives made from 1,4-cyclohexanedimethanol to toughen crystalline PET food trays. No improvements in crystallization rate or methods to improve the crystallization rate together with improvement of the impact performance are presented in these disclosures.
The use of impact modifying additives together with additives that improve the crystallization rate also is known. U.S. Pat. No. 4,713,268 discloses that the toughness of food trays when impacted at −18° C. (0° F.) is greatly improved by the addition of a core-shell impact modifier. These impact modifiers are produced by polymerization of monomers that yield a rubbery polymer upon polymerization and an alkyl methacrylate shell. The system described in the '268 patent also includes 1-5 weight percent aromatic polyester crystallization rate enhancer (with PBT being preferred) and 0-14.5 weight percent aromatic polycarbonate. U.S. Pat. No. 5,322,663 discloses that ethylene/methyl acrylate, ethylene/ethyl acrylate and ethylene/vinyl acetate copolymers may be used instead of linear low density polyethylene (LLDPE) as a nucleating agent in order to overcome plate-out problems associated with the use of LLDPE. The '663 patent does not provide any evidence regarding the effectiveness of these copolymers as nucleating agents. The '663 patent also teaches that these copolymers are effective impact modifiers at ambient (room) temperature but are not effective as impact modifiers at low temperatures. The '663 patent suggests that additional benefits would be derived by augmenting these impact modifying additives with those described in the '268 patent to improve low temperature performance.
U.S. Pat. No. 3,960,807 discloses the use of a “crack stopping agent” to improve the toughness of CPET trays. The crack stopping agent may be a polyolefin or other thermoplastic material present in concentrations of 2-16 weight percent. The tray compositions of U.S. Pat. No. 3,960,807 also contain 0.01-20 weight percent of an inorganic nucleating agent. U.S. Pat. Nos. 4,463,121 and 4,572,852 disclose that the inorganic nucleating agent employed in the compositions of the '807 patent are unnecessary and that the polyolefin also can serve as a nucleating agent. These patents also disclose that various crystallization aids often are added to the PET during the extrusion of the sheet material to control the crystallization rate of the PET.
Other poly(ethylene terephthalate) compositions containing tougheners and nucleating agents have been described previously. For example, in U.S. Pat. No. 4,357,268, Vanderkooi describes specific nucleating agents of acid salts that may optionally include impact modifiers. Coleman et al. in U.S. Pat. No. 4,448,913 describe polyester compositions optionally containing copolyetheresters. In these compositions, sodium benzoate is always present, and this may be assumed to be the nucleating agent, as the copolyesterether is specified as an additive to improve impact performance. Carson, U.S. Pat. No. 4,713,268, describes the use of core-shell impact modifiers comprised of an elastomeric phase composed of monomers that provide a rubbery polymer upon polymerization together with nucleating agents. These are used in conjunction with from 1 to 5% of a poly(alkylene terephthalate) crystallization rate accelerator, with the poly(alkylene terephthalate) having an alkylene group containing between 4 and 6 carbons including PBT. This patent does not disclose any unexpected benefit in crystallization rate due to the introduction of polyester with preformed core-shell impact modifiers, and does not discuss the use of PBT or copolyetheresters of PBT to increase crystallization rate with impact modifiers that do not consist of preformed particles.
U.S. Pat. No. 4,284,540 describes the use of ethylene/glycidyl methacrylate (GMA) copolymers as a toughening agent for polyesters when combined with 0.1 to 5 weight percent of a barium salt of a fatty acid. These barium salts are reported to broaden the window of crystallization temperature for injection molding applications. U.S. Pat. No. 4,753,980 discloses that polyester compositions containing 3 to 40 weight percent of either ethylene/ethyl acrylate/GMA terpolymer or ethylene/butyl acrylate/GMA terpolymer possess superior low temperature toughness when compared to analogous polyester compositions which contain an ethylene/methyl acrylate/GMA terpolymer. These compositions also may contain carboxylic acid crystallization rate enhancers with sources of sodium or potassium ion.
Published PCT Application WO 85/03718 discloses compositions containing ethylene/alkyl acrylate/glycidyl acrylate toughener additives. This publication discloses specifically acrylate copolymers with an ester substituent having an alkyl chain of from 2 to 8 carbon atoms, which does not include methyl acrylate copolymers. Compositions also are disclosed which include additional ingredients, including plasticizer or polyalkylene oxide “soft segments”, as well as a crystallization promoter. The amount of plasticizer and poly(oxyalkylene) “soft segments” together is at least 9% by weight of the matrix resin, and the ratio of soft segment and plasticizer can vary between 85:15 to 15:85. The addition of poly(oxyalkylene) “soft segments” together with toughener and 2-8% glass fibers demonstrated good impact performance, and were especially suited for certain end uses. No suggestion is made of the effect of poly(oxyalkylene) for crystallization rate improvement, and no synergistic effect for crystallization rate is suggested between the additives and the impact modifier.
Polyesters modified with polyolefin-based impact modifiers are known in the literature, but are not described in combination with the specific polyester or polyester copolymer additives of the present invention as having good impact performance as well as unexpectedly high crystallization rate.